Load cell

ABSTRACT

The invention is a load cell for the measurement of force, independent of displacement, created by means such as weight, acceleration, pressure, and the like. The load cell has a beam structure with beams positioned parallel to each other in at least one plane. Generally, these parallel beams are attached to a stationary base and have sensing means attached between the two beams. In operation, deflection of one beam creates a response in the sensing means due to the relative action or deflection in the second beam.

This application is a continuation-in-part application of U.S. Pat.application Ser. Nos. 862,827 and 863,162 both filed Apr. 3, 1992, nowU.S. Pat. Nos. 5,336,854 and 5,313,023.

FIELD OF THE INVENTION

The invention relates to a load cell for the measurement of force. Morespecifically, the invention relates to a load cell for the measurementof force resulting in strain or stress created by means such as, forexample, force, acceleration, or pressure converting the force into anelectronic signal transmitted to a point of computation or evaluation.The device resists environmental interferences such as those created byvariations in age, temperature and humidity which may ultimately affectmodulus, hysteresis, or anelastic material properties.

BACKGROUND OF THE INVENTION

Load measuring devices and cells are known in the art. For example,Gallo, U.S. Pat. No. 4,043,190, discloses a meter for measuring mass orforce wherein the sensed displacement acts indirectly on the tension ofthe two transversely vibrating electrically excited strings. Sette etal, U.S. Pat. No. 4,170,270, disclose an apparatus for preventing theoverload of a load cell used to measure deflection. Blawert et al, U.S.Pat. No. 4,237,988, similarly disclose an overload protection device forprecision scales. Paros, U.S. Pat. No. 4,384,495, discloses a mountingstructure for double bar resonators to ensure symmetrical loading of theresonator responsive to external forces. Also, Paros, U.S. Pat. No.4,751,849 discloses various mounting structures for use with forcesensitive resonators.

Further, Streater et al, U.S. Pat. No. 3,712,395, disclose a weightsensing cell which includes two differentially loaded vibrating members.Suzuki et al, U.S. Pat. No. 4,196,784, disclose a weighing scale havingan interior load cell. Great Britain Pat. No. 1,322,871 discloses aforce measuring apparatus having a pretension string which is excited toa state of transverse oscillation by an electronic circuit. Gallo, U.S.Pat. No. 4,300,648, also discloses a meter for sensing mass and forcecomprising two flat springs lying in a parallel plane. Pulvari, U.S.Pat. No. 3,274,828, discloses a force sensor based on piezoelectricoscillators.

Also, Reid et al, U.S. Pat. No. 3,366,191, disclose a weighing apparatuswhich relies on a bridge circuit. Norris, U.S. Pat. No. 3,479,536,discloses a piezoelectric force transducer which is a piezoelectricvibratory beam mounted to receive compressive and tensile forces alongits length. Agar, U.S. Pat. No. 3,529,470, discloses a force transducerhaving a composite strut with two bars which are to be maintained intransverse vibration at a common resonance frequency by electricalfeedback wherein the frequency of vibration indicates the force appliedto the composite strut. Corbett, U.S. Pat. No. 3,541,849, discloses anoscillating crystal force transducer. Wirth et al, U.S. Pat. No.3,621,713, disclose an instrument for measuring masses and forces whichwhen stressed by a load shows variation in frequency. Saner, U.S. Pat.No. 3,724,572, Van de Vaart et al, U.S. Pat. No. 3,853,497, Melcher etal, U.S. Pat. No. 3,885,427, and Paelian, U.S. Pat. No. 3,915,248, alldisclose a weighing system which functions by force or weight beingtransmitted to frequency sensitive elements. Meier, U.S. Pat. No.3,963,082, Wirth et al, U.S. Pat. No. 4,088,014, Jacobson, U.S. Pat. No.4,143,727, Ebbinge, U.S. Pat. No. 4,179,004, all disclose force sensingload cell.

Finally, Eer Nisse, U.S. Pat. No. 4,215,570, discloses a miniaturequartz resonator force transducer having the shape of a double endedtuning fork. Check et al, U.S. Pat. No. 4,239,088, disclose a scale withweight-to-period transducer which provides an oscillating output, theperiod of which varies as a function of the weight to be measured. Uedaet al, U.S. Pat. No. 4,299,122, disclose a force transducer based on avibrator having a pair of plate-shaped vibrating pieces parallel witheach other. Paros et al, U.S. Pat. No. 4,321,500, disclose alongitudinal isolation system. Eer Nisse et al, U.S. Pat. No. 4,372,173,disclose a resonator force transducer which includes a pair of elongategenerally parallel bars coupled at their ends with a double ended tuningfork arrangement.

Recently, quartz double-ended tuning forks have been used as forcesensors in environments where the tension resisted the movement of theloaded structure, or the tension was produced by strain within theloaded structure. Levered systems and parallel guiding structures havebeen used where the force applied to the force sensing crystal was afraction of applied load. The force sensing crystal was generally smallsince the force required to cause adequate frequency change in theresonant double-ended quartz tuning fork did not need to be great.

However, the loaded structure had to be massive to resist effects ofundesirable lateral deflection. The flexing portions of these structureswhich acted as parallel bending beams or bending fulcrums carried someload since the force sensing crystal and its bonded joints deflectedwhen tension was applied to the crystal.

The prior art load cells were dependent on the stability of the loadedstructure and the bonding joints, over temperature and time, for outputstability. For example, Albert, U.S. Pat. No. 4,838,369 discloses a loadcell intended to provide a linear relationship between the signalgenerated and the force sensed. Albert uses a specific crystal designattached by screws to the frame of the load cell which creates africtional joint resulting in inadequate zero return and cell precision.Albert relies on a longitudinally rigid structure to resistinterferences from varying load positions. The load cell of Albert isdesigned so that force expended on the load cell, when stressed, resultsin work or energy loss within the screw joints. In turn, this phenomenonalso results in poor zero return and precision. Without attention tomaterial similarly, non-strain sensitive designs, and reduction orcancellation of creep and hysteresis, Albert cannot provide a load cellwhich truly negates material and temperature effects.

Generally, material aging in these apparatus often caused long termperformance to suffer after calibration. Further, these apparatus werelimited in resolution by the degree in which anelastic creep and strainhysteresis were compensated for in their design. The quartz crystalbonding joints would often compensate for creep and hysteresis caused bythe loaded structure with their own counteracting creep and hysteresis.When the quartz crystals were bonded using adhesives such as epoxies,stresses were introduced in the glue joints and crystal because ofdifferential expansion between the base and the quartz and epoxyshrinkage during curing.

Further, as these stresses relaxed over time, the characteristics of thebonded joint changed because of the nonlinear stress-strain curve of theadhesive. This caused the load cell to have excessive zero and spanshift over time until the glue joint stresses had relaxed. Differentialexpansion between the quartz and the structural material would cause theforce sensor to have an output due to the temperature as well as appliedload.

As a result, a need exists for a load cell which can compensate forchanges in modulus of elasticity, anelastic creep, and strain hysteresisoccurring in the elements of the cell due to stresses created by theenvironment of application.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a force sensing loadcell comprising a base, a load bearing element affixed to the base,means for supporting capacity affixed to the base and spaced apart from,and parallel in at least one plane to the load bearing element, andmeans for sensing force, the force sensing means affixed between theload bearing element and the capacity supporting means. The load bearingelement functions as a spring. A spring is an element which may storeenergy through deflection when a force does work on the moveable portionof the spring and which may do work by returning stored energy byproviding a force moving through a distance.

In accordance with a further aspect of the invention, there is provideda force sensing load cell comprising a three-dimensional structurehaving an interior opening defined by an upper wall, a lower wall, andfirst and second side walls, a base affixed to at least one wall withinthe interior opening of said three-dimensional structure, a load bearingelement affixed to the base, means for supporting capacity affixed tothe base, and spaced apart from, and parallel in at least one plane, tothe load bearing element, and means for sensing force, the force sensingmeans affixed between the load bearing element and the capacitysupporting means. Further embodiments may comprise more than one forcesensing means affixed between any number of load bearing elements.

In accordance with another aspect of the invention, there is provided aforce sensing load cell comprising a first and second electricalelements for sensing force, a base, and means for structurallysupporting the two electrical elements affixed to the base and providingequal magnitude but opposite influence on the first and secondelectrical elements when the load cell is stressed. Upon stressing theload cell through the incidence of force, independent signal processingoccurs of the first and second electrical elements producing anindependent mode signal separate from a differential mode signal.

All of these embodiments may comprise means for sealing the load sensingelements whereby they are protected from environmental effects. Theseembodiments may further comprise stopping means for limiting the rangeof flexure of the sealing member. Additionally, in order to avoidinterfering vibrations from various electrical leads, the contact wiresto and from the force sensing crystals may be treated to otherwisedampen the vibrations received from the crystals to which the wires areattached. In accordance with a further embodiment of the invention thereis provided means to avoid structural overload and breakage of the loadcell of the invention such as nonmetallic or nonmetallic coatedfasteners for attaching the load cell to its mount wherein the fastenerwill absorb the shock of sudden forces. Other breakage avoidance meansmay comprise a resilient shim placed between the load cell and itsmount.

In accordance with one preferred aspect of the invention, there isprovided a force sensing load cell comprising a three-dimensionalstructure having an opening defined by an upper wall, a lower wall andjoined by first and second side walls, a base positioned within saidopening affixed to at least one of the opening walls, a first capacitysupporting cantilever beam, said first capacity supporting cantileverbeam affixed to said base and extending within the plane of thethree-dimensional structural opening, a second parallel capacitysupporting cantilever beam, affixed to the base also extending withinsaid three-dimensional structural opening, a load beam affixed to thebase intermediate between the first capacity supporting cantilever beamand the second capacity supporting cantilever beam, a first electricalsensor affixed between the first capacity supporting cantilever beam andthe load beam, and a second electrical sensor affixed between the loadbeam and the second capacity supporting cantilever beam. Upon stressingthe load cell through the incidence of a force, independent signalprocessing of the first and second electrical sensors produces anindependent mode signal separate from a differential mode signal.

The invention provides a force sensing load cell, which develops outputsignals highly isolated from unwanted information and from disturbancesattributable to variations in the location of the loading forces. Theforce sensing cell displays reduced anelastic creep and static strainhysteresis effects. The design of the force sensing cell enables thecell to display reduced effects due to temperature on zero, span, andprestress in the assembly at elevated temperature.

Preferably, the structure is machined monolithically from an isotropicmetal, therefore, the modulus of elasticity is nearly homogeneous, andthe effect of the elastic modulus is nearly cancelled if thecrystal-glue system is very stiff relative to the various elements ofthe cell which are in series with the crystal. This means that the loadcell may be machined from any reasonably homogeneous structural materialwith a fairly well behaved elastic modulus with close to the sameperformance, if designed within the limitations of the material used.

Anelastic creep, static hysteresis, elastic modulus temperaturesensitivity, and return to zero load reading effects on the forceapplied to the force sensing crystal, all tend to cancel out by thecommonality of these effects. For example, if the load cell elementresisting the force, as well as influencing the crystal, should haveanelastic creep, the output will normally increase over time. However,with the invention, the cell element in series with the crystal also hasanelastic creep and will cause the output to decrease, thus cancellingthe anelastic creep effect on the force applied to the crystal.

Zero shift is reduced because the spring loaded by the crystal has alarge deflection relative to the differential expansion between thequartz and the structural material. Zero shift may also be cancelled ifa second oppositely loaded crystal is used in the same monolithicstructure with a second spring loaded by it which is matched physicallyto the first, and its output is subtracted from the first crystalsoutput.

Span shift may also be curtailed and almost cancelled because theelastic moduli of both the parallel spring system and the series springsystem have very nearly the same sensitivity to temperature, and as theparallel springs deflect more under the applied load, the reactive forcein the series spring does not change because of this increase.

Aging effects due to relaxation of elevated temperature cure prestressin the bonding epoxy is reduced on zero because initial zero shift dueto differential expansion between the quartz and the structural materialis small, relative to the series spring deflection. This movement due torelaxation is small and may be cancelled as in zero shift overtemperature with a second crystal and spring if glue joints areconsistent.

Aging is also reduced on span because the deflection under load of theglue joints is very small relative to the deflection of the seriesspring. Therefore, the elastic modulus change in the glue due torelaxation of the prestress due to the elevated temperature cure, hasvery little effect.

With regard to loading effects, anelastic creep is reduced because thecontinued movement over time of the parallel spring loaded by theapplied load, is essentially cancelled by the relaxation of the reactiveforce in the series spring due to the isotropic behavior of the springstructures.

Further, static strain hysteresis is reduced because the movementresistance in the parallel and series spring are the same due to theirisotropic behavior.

Zero return after load removal is affected in the same way ashysteresis. Span sensitivity to load position is reduced by theprinciple of the shear induced bending of the parallel spring whenloaded toward either end away from the load cell center. Spansensitivity to a load positioned to either side is also reduced becausethe top and bottom lateral flexures bend easily sideways relative to thehorizontal flexured parallelogram elements in the outside structure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view of a load cell in accordance with onepreferred embodiment of the invention.

FIG. 2 is a side plan view of the load cell depicted in FIG. 1.

FIG. 3 is a top cut away sectional view of the load cell shown in FIG. 2taken along lines 3--3.

FIG. 4 is a perspective view of a load cell in accordance with analternative embodiment of the invention.

FIG. 5 is a perspective view of a load cell in accordance with a furtheralternative embodiment of the invention.

FIG. 6 is a top plan view of quartz double-ended tuning fork transducerin accordance with the invention.

FIG. 7 is a top plan view of an alternative embodiment of a quartzdouble-ended tuning fork transducer in accordance with the invention.

FIG. 8 is a side plan view of one embodiment of the lead cell of theinvention seen in FIG. 2 additionally depicting a sealing membrane andstop means.

FIG. 9 is a partial cutaway side plan view of the load cell of theinvention depicted in FIG. 9 in which sealing membrane and stop meanshas been cut away.

FIG. 10 is a side plan view of the load of FIG. 1, showing sealing meansincluding sealing compound introduced into the opening of the load cell.

FIG. 11 is a graphical depiction of performance data received fromstandards conformance testing of the load cell of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to the Figures wherein like parts are designated with likenumerals throughout several views, there can be seen a force sensingload cell 20 shown in FIG. 1, 2 and 3. The load cell may generallycomprise a three-dimensional structure having an opening defined by anupper wall 26 and a lower wall 28, joined by first and second sidewalls, 24A and 24B, respectively. The first and second side walls 24Aand 24B, respectively, are defined by first and second structuralmembers 23A and 23B, respectively. The cell 20 generally comprises abase 40 positioned within the opening 10 and affixed to at least one ofthe opening walls. Affixed to the base is a first capacity supportingcantilever beam 42A which extends within the plane of saidthree-dimensional structural opening 10, as well as a second capacitysupporting cantilever beam 42B spaced apart from, and parallel to thefirst capacity supporting cantilever beam 42A. In between the first andsecond cantilever beams, 42A and 42B, a load beam 45 also is affixed tothe base and may span up to the upper wall 26. Two sensors are alsoaffixed to the various beams in this embodiment of the invention. Thefirst sensor is affixed between the first capacity supporting cantileverbeam 42A and the load beam 45. The second sensor is affixed between theload beam 45 and the second capacity supporting cantilever beam 42B.

Turning to the simplest embodiment of the load cell of the invention,FIG. 5 shows a load cell which generally comprises a base 40 whichprovides stationary positioning of all elements in the invention. Theload cell base 40 may, in accordance with certain embodiments of theinvention, provide a base for receiving weight, force or otherdisplacement from the load to be sensed. Among other functions, the basemay also function as a platen or other surface for receiving the forcewhich is to be analyzed.

Generally, the base 40 may comprise any number of designs and substancesas long as a certain flexibility is provided. The base must be capableof deflection so as to transmit forces, sensed by the base, to theparallel beams 42 and 45 attached within the base. Through this force,the parallel beams ultimately distribute stress and strain to thesensing means suspended between the two parallel beams.

Preferably, the load cell comprises homogenous and isotropic metal. Theload cell is defined as a unitary or monolithic structure wherein thebase and parallel beam structure are molded as one continuous unit. Thismay be done through any number of means including machining, milling,ion cutting, casting or any other means known to those of skill in theart. Preferably, the load cell is stress relieved after each millingcycle. Further, in the more preferred embodiments of the invention (FIG.1), the load cell is preferably machined symmetrically and the springconstants of the beams 42A and 42B matched. To this end, the response ofthe sensing elements should be matched as closely as possible. Further,the load cell may be further milled and stress relieved by stressing theload cell, gauging the response and relieving excess material from thestressed cell to equalize the response. Preferred compositions includemetals such as, for example, elemental metals and metal alloys. Metalcompounds including aluminum and its alloys such as 2024-T3, 7075-T6,and 1100; copper and its alloys including ASTM B147, ASTM B145, and ASTMB146; zinc and its alloys including ASTM A40A, and ASTM AC41A, as wellas any other metals that are known to provide a light weight structurehaving good resilience to the forces intended to be sensed by the cell.Most preferably, metals such as aluminum and its oxides are used informing the load cell of the invention but almost any structuralmaterial which lends itself to manufacturability may be used.

The load cell may also be made from polymer systems which provideuniform material characteristics, that is modulus, temperaturesensitivity, expansion characteristics, etc. Plastics such aspolyamides, polyamide-imides, polyvinyl chloride, polyethylene,propylene, polycarbonates, aminoplasts such as melamine resins, castepoxy resins, cast epoxy resins, cast acrylics, cast fluoroplastics,phenolics, polyacrylonitriles, cast polyurethanes, cast polyesters orpolyolefins; synthetic or natural rubber polymers and copolymers such assilicones; ceramics such as silicon dioxide; and cellulosic products; ormixtures of any of these compounds.

The simplest embodiment of this base 40 can be seen in FIG. 5 as a rigidfixture to which a parallel beams 42 and 45 are attached. In analternative embodiment, FIG. 4, the base 40 may be seen as a mountingplate for positioning of the load bearing means or load beam 45 andcapacity support means 42. To this end, the base assists in positioningload beam 45 and cantilever support beam 42 parallel to each otherwithin at least one plane, FIG. 5.

As applied in a load cell, FIG. 4, the base 40 may be used to positioncantilever support beam 42 and load beam 45 in the opening 10 of theload cell structure 20. More specifically, the base 40 extends generallyand is attached to interior wall 28 which forms the load cell opening10. While not essential, the load cell base 40 may be attached tointerior wall 28, or for that matter any of the other interior wallsincluding side wall 24A, side wall 24B or upper wall 26 through anynumber of appendages including flexure 32.

Thus, as can be seen, the base 40 may take any number of forms includinga simple rigid structure 40 as seen in FIG. 5 or for that matter themore complex platform 40 as seen in FIG. 4. The base 40 may alsocomprise flexures between the various beams mounted on its surface toprevent undesired or interfering movement out of the intended axis andto adjust leveraged effects.

The load cell of the invention generally also comprises a parallel beamstructure which assists in the measure of force incident to the loadcell. The parallel beam structure 42 and 45 (FIGS. 4 and 5) alsofunctions to hold the sensing means 52. Generally, the parallel beamstructure may comprise any configuration of the appropriate material anddimension which will facilitate and exhibit deflection transfer underthe intended conditions.

Turning back to FIG. 5, definition of parallel beams 42 and 45 willdepend on the magnitude of the force, acceleration or other movement tobe sensed by the beam structure. Relevant parameters include the length,width, and thickness of the parallel beams, the necessity of the beam inhaving an open insert 44 and 46, FIG. 5. Also relevant are the materialsused to create the beams and the presence of flexures to attach thebeams to any intended base.

Generally, the parallel beam structure may comprise any number ofdifferent configurations in accordance with the invention. Onealternative embodiment of the parallel beam structure can be seen inFIG. 5 comprising parallel beams 42 and 45. In this instance, parallelbeam 45 serves as a load beam, being the primary support of any mass,force, or other displacement placed in the structure. In the meantime,load beam 42 serves as means for supporting the capacity affixed to thislower beam 42. In other words, beam 42 will receive the principal forceresulting from displacement transferred through element 52. In themeantime, beam 45 serves as a load bearing element as well as anadditional station to seat sensing element 52.

As can be seen in FIG. 5, interior portion 41 results from the lowerside of beam 45, the upper side of beam 42, the interior side of sensingmeans 52, and the exterior side of base 40. Upon deflection, parallelbeams 45 and 42 will move in an axis which is consistent with thedirection of the displacing force. However, the exterior surface of base40 as well as the interior surface of sensing means 52 will remainparallel to each other or in "parallelogram" configuration. The resultof this parallelogram structure is the negation of a moment arm in theforce sensor.

This configuration produces a load cell which is easily manufactured toprovide a well behaved response regardless of where force is appliedgenerally across the surface of the end of parallel beams 45. Further,the parallel beam structure, including the close proximity of the beamsto one another, provides a structure wherein changes in temperature,humidity, as well as other environmental stresses result in beams whichrespond similarly to one another. In essence, the invention provides aforce sensor which compensates for changes in modulus and allows for anyvariation and deflection created by environmental stresses.

A more preferred alternative embodiment of the invention can be seen inFIG. 4 wherein load bearing member 45 is positioned parallel, within theopening of the three-dimensional structure or block, to cantilever beam42 or the capacity supporting means. Here again, the base 40 has aninterior plane which is parallel to the interior side of the sensor 52.At the same time, the overall configuration of cantilever beam 42 isparallel at its interior edge with the interior or opposing face of loadbeam 45.

While not essential, openings 44 and 46 may be defined in each of theload beams 42 and 45, respectively. These openings allow greateradjustment of the sensitivity to force allowing for the load beamdeflection to be created by a preferred magnitude of force. In essence,openings such as those found at 44 and 46 allow for the creation of aload cell having greater adjustment to the force incident on the cell.The openings easily bored or machined with standard tooling, may beslotted or dumbbell-shaped.

Generally, as can be seen in FIG. 4, a load cell may take any number ofconfigurations including that of a three-dimensional six-sided block.Within the cell, there may generally be an opening 10 defined by the twoside walls 24A and 24B as well as an upper side wall 26 and a lower sidewall 28 positioned within the opening is the base 40 on which is mountedload beam 45 and cantilever capacity supporting beam 42.

Optionally, any number of elements within the load cell may be attachedthrough the use of flexures. Flexures assist in determining the loadcapacity of the parallel beam structure as well as preventing the baseor other structures from pivoting or bending into a plane outside thatintended. Flexures are integral in converting the force to be sensedinto displacement of the base and parallel beam structure so as toinfluence the sensor by a mechanical action ultimately resulting in atransduced signal from the sensor.

Generally, flexures may be positioned anywhere within the load cell toprevent interfering deflection. Specifically, as can be seen in FIG. 4,a flexure 32 may be found as a part of base 40 attaching base 40 tolower wall 28. Flexure 34 may be found at the top of load beam 45attaching load beam 45 to upper wall 26.

Within the opening of the three-dimensional structure, sensing element52 is supported between load bearing beam 45 and capacity supportingbeam 42. Load beam 45 and cantilever support beam 42 are in parallelwithin at least one plane within the opening 10 of three-dimensionalblock 20. This maintains the parallelogram-like structure created bybase 40, sensing element 52, as well as the two interior or opposingside walls of beam 42 and beam 45. Accordingly, deflection of the loadcell by any force will result in a parallelogram-like response withinthe invention.

The sensors may be attached through means which will provide an integralor fixed and stable joint such as thermoplastic or thermosettingadhesives. One preferred class of adhesives includes epoxy-typeadhesives, such as those commercially available sold under the brandname ECCOBOND available from Emerson and Cummings. Preferred load cellperformance may be as rigid and stable as possible. To minimize jointeffects a larger deflection of the parallel spring system is desired.Then when the attachment joints move, this movement is small relative tothe beam deflection. The output will then be less sensitive to a smallamount of deflection due to less than perfect attachment joints.

The load cell of the invention also comprises sensing means 52, FIG. 5.The sensing means generally function to sense force created by theincidence of a force on the load cell. The sensing means is influencedby the force either of compression or tension and transduces this forceinto an electrical signal which is sent to a circuit for evaluation.Generally, any number of sensing means may be used in accordance withthis invention including hard electrical wiring, electrical circuitry,transistor circuitry, including semiconductors and the like. Sensingmeans which may be used include optical, electro-mechanical, solid stateand impedance or resonator sensing means.

One preferred sensing element comprises an impedance or resonator suchas a quartz crystal. Preferred resonators include those available fromMicrocrystal made by ETA of Grenchen, Switzerland. Other resonatorswhich are also useful are those disclosed in Eer Nisse et al U.S. Pat.No. 4,724,351, Eer Nisse et al U.S. Pat. No. 4,215,570, Norling U.S.Pat. No. 4,912,990, Eer Nisse et al U.S. Pat. No. 4,372,173 all of whichare incorporated herein by reference.

Such resonators are commonly referred to as a double-ended tuning fork52 and generally comprises two parallel tines 54 joined together attheir ends 56 which may also be held in place on the load cell by endpads 53, FIGS. 6 and 7. The tines may be excited piezoelectrically so asto vibrate in an amount bending them in opposition to each other withinthe plane of the plate. By applying a tensile or compressive force tothe crystal along its longitudinal axis, its resident frequency willincrease or decrease like the strings of a violin.

Any number of transducers may be used in accordance with the invention.As can be seen in FIG. 6, we have found that double-ended tuning forkshaving an overall thickness of about 0.175 mm, an overall length (L) ofabout 15 mm, tine 54 lengths (l₁) of about 5.26 mm, pad widths (W_(p))of about 2.9 mm and pad lengths (l₂) of about 2.5 mm. Generally, thedistance between the tines (d) is about 0.16 mm and the width of thetines (W_(t)) is about 0.26 mm. The length of tine end regions (l₃) isabout 1.17 ram, while the contacts have a length (l₄) of about 0.6 min.An open area 55A and 55B may be seen in either end pad having a lengthof about 1.0 mm and a width of about 0.4 mm in 55A and 0.49 mm in 55B.Open areas 55A and 55B start about 1.0 mm from either end of theresonator. Between the end pads 53 and contacts 58 there is a void whichmay have a length (l₅) of about 0.6 mm with the width (W_(v)) of theresonator at this void behind about 1.2 mm. The overall length (l₆) ofthe resonator between end pads is about 10.0 mm. While any number ofmaterials can be used to manufacture the transducer, piezoelectricmaterials have been found useful and alpha quartz has been found to bepreferred. Further, in order to provide quartz crystal resonators withthe desired thermal stability the crystals are cut on the Z plane.

An alternative quartz force transducer may be seen in FIG. 7 which isalso useful in the invention. The transducer depicted in FIG. 7 has anoverall thickness of about 0.150 mm and is comprised of quartz cut alongthe Z plane. As shown the overall length of the transducer (L) is about7 mm, (l₁) being about 3.2 mm, (l₂) being about 1.5 mm, (l₃) being about0.4 mm, (l₆) being about 4 mm, (l₇) being about 0.6 mm, (l₈) being about0.4 mm, and (l₉) being about 0.5 mm. The width of the tines (W_(t)) isabout 0.18 mm, d is about 0.1 mm, and (W_(p)) is about 1.9 mm.Additionally, electrodes 59 formed of gold may be positioned at eitherend of the transducer on either side of openings 55A and 55B. Thispositioning may also be used on the electrode shown in FIG. 6.

Generally, the quartz resonators may have gold electrodes distributed ontheir surfaces to cause the crystals to interact with a resonatorcircuit as a frequency controlling element when the circuit is connectedto the crystal electrodes.

The quartz crystal is a very staple and reliable electromechanicaldevice. The force-frequency response and quasidigital output signal ofthe device are associated with accurate frequency measuring capabilitiesand enable good performance. Outstanding mechanical and physicalproperties of single crystal quartz yield a behavior with tightrepeatability and without hysteresis as well as good thermal and longterm stability. Furthermore, only small displacements are induced in themounting structure due to the high stiffness of quartz.

In practice the transducer may be attached between either cantileverbeam and the central load beam 45. Any number of attaching mechanismsknown to those of skill in the art may be used including solder, weldingand adhesive among others. One preferred adhesive includes epoxy soldunder the brand name ECCOBOND and available from Emerson & Cumming, Inc.

An oscillator is needed in order to drive the quartz resonator. Sinceequivalent electrical parameters of the crystal are similar to those ofthe widely used tuning forks, oscillators known to those of skill in theart are adequate for operation of the crystal. Oscillators usingstandard integrated amplifiers are easy to implement. A usefuloscillator circuit may be supplied by 3 to 15 volts through any varietyof circuit configurations known to those of skill in the art.

The frequency range of the transducer may vary depending upon the givenapplication. However a frequency of 20 KHZ to 100 KHZ, preferably 44KHZ, 48 KHZ, and most preferably 86 KHZ to 94 KHZ has been found mostuseful.

The load cell of the invention may also comprise any variety ofcircuitry useful in evaluating the electrical signal received from thesensing means and reflecting the appropriate magnitude of the sensedforce. The electrical signal is transmitted from the load sensing meansto the circuitry through leads 60, see FIGS. 8 and 9.

Generally, any circuit commensurate with this purpose which will providea linear response to an electrical signal may be used in accordance withthe invention.

Preferably, circuitry found useful with this invention are impedancecircuits such as Wheatstone bridge configurations and the like, ordifferential circuits which cancel the bias signals of the elementswithin the load cell. The Wheatstone bridge uses four resistive elementsarranged in a square circuit with voltage applied across two diagonalcorners and the signal measures across the other diagonal corners.

In order to reduce vibration and interferences with the vibration of thequartz crystals when the load cell is in either the stressed (subjectedto a force) or unstressed state the leads 60 may be coated, FIGS. 8 and9. This vibrational interference may adversely effect the electricalcharacteristics of the load sensing means 52a and 52b. Placing a thincoating of material on the leads 60 will tend to dampen vibrations andminimize their adverse effect on the load sensing means 52a and 52b.

The leads 60 may be coated with any number of nonorganic or organic aswell as thermoplastic or thermosetting systems effective in dampeningthe vibrational frequency within the leads. Coating systems which may beuseful to this end include natural or modified polymers such as dryingoils, cellulose esters, and cellulose ethers or combinations thereof;condensation system coatings such as alkyd resins, polyesters, aminoresins, phenolic resins, polyamides, polyurethanes, epoxy resins,silicones, or mixtures thereof; vinyl polymers and copolymers based uponmonomers including butadiene, acrylic or methacrylic esters, vinylacetate, vinyl chloride, vinylidine chloride, styrene, vinylacetel orbutyryl, fluorocarbons, or mixtures thereof; as well as resincombinations of any of the listed components.

Preferably, the coating material comprises silicone applied at a coatingthickness ranging from about .001 to .005 inches. This thickness mayvary depending on the method used to coat the leads. Lead coating may beimplemented by any number of means know to those of skill in the artincluding manual brushing of leads. One preferred means of coating mayinclude the pneumatic application of dampening material through avolumetric device sold as 1000XL-PI available from EFD of Providence,R.I.

Returning to FIG. 1, a more preferred embodiment of the parallel beamstructure can be seen. Essentially, this embodiment of the load cell mayfunction with two sensing elements 52A and 52B, a base 40, and adequatestructure to ensure that both sensing elements will be influencedequally when the load cell is stressed. This will allow for theindependent signal processing of each respective sensor producing commonmode signal effects and differential mode signal effects.

Common mode signal effects include the effects of temperature, pressure,external vibration and aging among any other effects which influenceboth cantilever beams 42A and 42B as well as sensors, 52A and 52B,equally, FIG. 1. Differential mode effects are most importantly anyforce or stress on the sensor which influences the cantilever beams 42Aand 42B as well as sensors 52A and 52B, unequally.

In this case, hollowed cantilever beams 42A, 42B and 45 are attached toa single unitary base 40 stemming from the opening lower side 28. Theload cell is able to compensate for changes in the modulus ofelasticity, variations and hysteresis as well as anelastic creep throughattachment of both of the flexible beams 42A and 42B through sensingmeans 52A and 52B attached between the upper ends of the flexible beamsand the load bearing beam 45. In this instance, any change in modulus,hysteresis or creep will be cancelled by attaching the sensing meansbetween the flexible arms and the load bearing beam as both will beaffected proportionally.

While not wishing to be bound to a specific mode or theory of operation,we believe that load cell operation is focused between a load bearingbeam 45 arranged with a bridging gap to a small spring 42. This smallspring 42 bears a load because of the deflection of the load bearingbeam 45 by force transfer through a relatively rigid force sensor 52A or52B deflecting only, for example, about 0.00005 inch at full load. Inthis case, the entire load cell may deflect only about 0.015 inch. Theforce sensor then experiences a force which is essentially independentof the elastic modulus of the machining material.

In this case, where P_(T) is load, the total load born by the parallelsprings is:

    P.sub.T =P.sub.l +P.sub.2

where P₁ is the load born by springs 45 and P₂ is the load born byspring 42.

The load on each beam (spring) is proportional to its deflection:

    P.sub.1 =K.sub.1 Y.sub.1 and P.sub.2 =K.sub.2 Y.sub.2

where Y₁ and Y₂ are respective deflections in inches, K₁ and K₂ arerespective spring constants representing pounds of load per inch ofdeflection.

If the connecting force sensor has a very large spring constant then thetwo beams have essentially equal deflection under load.

    Y.sub.1 =Y.sub.2 and P.sub.1 /K.sub.1 =P.sub.2 /K.sub.2

The spring constant of each beam is proportioned to the modulus of itsmaterial of composition:

    K.sub.1 =C.sub.1 E.sub.1 and K.sub.2 =C.sub.2 E.sub.2

where C₁ and C₂ are constants dependent on the beam shapes and E₁ and E₂are their respective elastic moduli.

Because the material of both springs is the same, their moduli are thesame. ##EQU1## The force on P₂ is equal to the force on the force sensorbecause the force sensor is the connecting element. Therefore the sensedforce is proportional to the applied load. ##EQU2##

C₂ and C₁ are dimensional factors so P₂ is essentially directlydependent on the applied force without substantial modulus effects.

Therefore, modulus sensitivity to temperature, anelastic creep (a timedependent modulus effect) and static hysteresis (an internal materialfriction effect which creates a history dependence of modulus) becomenegligible if the structures in both springs see similar environmentaleffects and stress levels when a nonlinear stress-strain relationshipexists.

The output signal of these load cells is almost purely dependent ontheir structural dimensions and the applied load, if temperature doesnot effect the force sensor's performance. When the load cell is notmade from the same material as the force sensor, temperature changeswill cause a change in the force sensors signal in the form of a zeroreference shift. Other environmental effects such as barometric pressuremay also cause similar effects on the zero stability. To overcome theseenvironmental effects a closely matched second force sensor is generallypreferred. The second force sensor 52B may be mounted between the loadbeam 45 and another parallel beam 42B. This force sensor will then see anegative force as compared to the first sensor 52A. By extracting thedifference between the two force sensors, the output due to the appliedforce is doubled, but interference effects which affect both sensors 52Aand 52B equally cancel.

In order to further improve the accuracy and precision of the load cellof the invention, a sealing means may be provided. Any number of sealingconfigurations may be used such as exterior coating of the load cell,filling the load cell cavity with sealing compound, or providing sealing"windows" on the front and back of the load cell interior opening 10.For example, the load cell may be completely wrapped to isolate theinternal works of the cell. Additionally, the transducers may beencapsulated and the entire cavity filled to isolate the various beamsagainst unwanted vibrational interferences. Encapsulation of thetransducers will prevent them from being damped during their normaloperation. Referring again to FIG. 8 and FIG. 9, a first and secondsealing member, 61 (shown partially cutaway by edge 61') and 62,respectively, provide the sensing means 52A and 52B with additionalprotection from the environment. The sealing member may take any formwhich will isolate the internal load sensing elements of the cellwithout interfering with their function. Preferably, the sealing membersconform to the shape of the cell opening and adhere to the first 24A,second 24B, upper 26, and lower 28 walls of the load cell. The firstseal 61 is adjacent to a first edge 63 of the opening such that it islocated on a first side 64 of the load sensing means 52A and 52B. Thesecond seal 62 is adjacent to a second edge 65 of the opening such thatit is located on a second side 66 of the load sensing means 52A and 52B.

The sealing members may be formed to surround the leads 60, which can beseen to penetrate 67 the second sealing member 62, FIG. 8. The secondsealing member 62 adheres to the side of the load cell and thepenetration of the sealing member by leads 60 does not reduce theenvironmental protection offered by the seal.

Both the first 61 and second 62 sealing members may be substantiallythin and flat. The thin, flat shape of the seals precludes contact ofthe seals to the load sensing means 52A and 52B. Generally, the sealingmembers may comprise a thickness which may range from about 0.1 to 1inches, preferably about 0.1 to 0.2 inches, and most preferably about0.125 to 0.1875 inches. Preferably, the sealing means may comprisefilling substantially all or the entire load cell cavity FIG. 10. Inthis case, the transducers isolated from the sealing means so as not tointerfere with the vibrational operation of the transducers.

In production, the load cell of the invention may be sealed bypositioning the load cell against a flat, nonadherent surface such as asurface treated with flouropolymers like TEFLON®. In this context flatmeans that the load cell opening 10, (See FIG. 1), is sealed on one sideby the supporting teflon surface. Sealing compound may then beintroduced into the opening 10 to seal the side of the opening 10adjacent the teflon surface. As explained earlier, the sealing membermay have a varying thickness depending on the overall dimensions of theload cell. However, the sealing member should preferably not be so thickas to foul or otherwise interfere with the transducers. Once the firstsealing member is formed, (and cured if required by the materials used),leads 60 may be inserted through the seal. The load cell may then beinverted, to position the opposite side of the opening n the Teflon®surface. The same procedure may then be completed for completing thisseal. Once means of completing this seal comprises the use of anindustrial syringe, by inserting the needle through the seal previouslyformed, and introducing the desired volume of sealing compound.

Once the second seal has been formed the opening 10 of the cell may befilled with sealing compound up to a line 72, (See FIG. 10 which shows aload cell having sealing members without plates 69), beneath thetransducers 52A and 52B. By sealing the internal components in the loadcell opening 10, interfering vibrations are reduced thereby avoidingactivity dips in the performance of the load cell.

Here again, the sealing conformed may be introduced into the opening 10between the two sealing members to fill the opening up to line 7 whichmay be positioned so as not to interfere with the transducers. Opening10 is then reduced to volume 75, which even further reduces the quantityof interfering vibrations in the cell. Even more preferably, the cellmay be inverted and filled with sealing compound to a line 78 which willeven further reduce volume 75 and, in turn, reduce interferingvibrations. Overall the volume of sealing compound used to fill theopening 10 of the cell may be that necessary to fill the opening withoutcontacting the transducers 52A and 52B.

The sealing members covering the internal load sensing constituents ofthe cell of the invention may comprise any number of compositionsapplied to the load cell through any number of means. Load cell sealingmembers may comprise any number of systems effective in sealing theinternal area of the load cell. Preferably, the sealing means comprisesa material having high thermal stability which not susceptible tochemical or physical aging. Preferably, the material also has a lowmodulus with 3 to 9 mm penetration using a Universal Penetrometer with a19.5 gram shaft having a 1/4 inch diameter foot. Sealing members may beinorganic or organic, thermoplastic, thermosetting. Metal alloy filmsmay also be used including alloys of aluminum, nickel, zinc, molybdenumand the like. Sealing systems which may be useful to this end includenatural or modified polymers such as drying oils, cellulose esters, andcellulose ethers or combinations thereof; condensation system coatingssuch as alkyd resins, polyesters, amino resins, phenolic resins,polyamides, polyurethanes, epoxy resins, silicones, or mixtures thereof;vinyl polymers and copolymers based upon monomers including butadiene,acrylic or methacrylic esters, vinyl acetate, vinyl chloride, vinylidinechloride, styrene, vinylacetel or butyryl, fluorocarbons, or mixturesthereof; as well as resin combinations of the listed chemicals.

The sealing means may comprise a precast or precured film, a film whichis cured after application, or a material used to fill the interioropening which cures after application made from any of the materialsprovided above. Preferably, the coating material comprises siliconecompounds. Silicones generally have a backbone comprising silicon andoxygen with an organic group attached to the backbone such as an alkylgroup, flouroalkyl group, a phenyl group, a vinyl group, hydrogen, ahalogen group, an alkoxy group, an acyloxy group, an alkylamino group,or mixtures thereof. Silicone systems useful in the invention maycomprise organic and inorganic additives, fillers to increase strength,or solvents as well as additives to promote flexibility, temperaturestability, prevent aging, antioxidants, adhesion promoters, andextenders.

Silicone polymers may comprise silicone homopolymers, silicone randompolymers, or silicone organic block copolymers. Silicone polymers foundparticularly useful are those which vulcanize or cure to increase inmolecular weight either from a one-part or two-part system. Two-partsilicone systems generally comprise a catalyst as one part and theuncured silicone as the second part, which may be present in any numberof curable forms including alkoxy-, amino-, ketoxy silicone, as well asother silicones. The catalyst may be any catalyst appropriate to promotecuring known to those of skill in the art through curing mechanisms knowto those of skill in the art. Most preferably the compounds used inaccordance with the invention are those available from Loctite ofNewington, Conn., LOCTITE® 5491 in a two-part silicone composition whichcures when combined.

The sealing members may have a tendency to flex in the use of the loadcell and may come into contact with the load sensing means 52A and 52B.This fluctuation may be dampened by one or more plates 69 affixed to thefirst seal such that it is supported by the seal and it does not touchthe first wall, second wall, upper wall, or lower wall. A second platemay be similarly attached to the second seal.

These plate members may be comprised of any number of hardened elementalplastics such as thermoplastic or thermosetting compositions such asacrylics, epoxies, alkyd compositions, polyesters, cellulose ester andether compositions, urethanes, phenolics, vinyl chlorides, or mixturesthereof.

Further, the plate members may also comprise elemental metals or metalalloys such as gallium, silicon, mercury, arsenic, tellurate, aluminum,or alloys thereof. Other metals which are useful include cobalt, nickel,iron, chromium, niobium, tungsten, tantalum, vanadium, and alloysthereof. Also useful are metals and metal alloys of silver, gold, zinc,iron, platinum, manganese, magnesium, tin, exanthem, indium, titanium,and binary alloys, ternary alloys, and quaternary alloys, for example.

Generally, the preferred alloys include alloys of aluminum, such as6061, 2024, 3003, or 5005.

In practice, the thickness of the plate members may range from about0.010 to 0.060 inches, and preferably about 0.020 to 0.030 inches. Theplate members may take any conceivable design consistent with theirfunction in the load cell of the invention. To this end, plate membersmay be patterned or otherwise weighted so as to provide a plate ofappropriate structure for the intended function. The overall area of theplates as a percentage of the interior of the load cell may generallyrange from about 0.0 to 90%, preferably from about 50 to 75%.

In use, the load cell of the invention is preferably placed within anenvironment or application wherein the cell is stressed through theapplication of a force. In any given application, the load cell usedwill be gaged to have certain tolerances in order to provide adequateaccuracy and precision in sensing the force. When these tolerances areexceeded, it may be possible to disable the load cell through theapplication of forces which exceed these tolerances. The occurrence ofbreakage modes or forces which otherwise exceed the tolerances of theload cell may occur in at least two forms. The first form is theapplication of a force which exceeds the tolerance of the load cellthereby overloading the cell and causing a rupture of the quartzcrystals or an inelastic displacement of various constituents of thecell.

A second breakage mode which may occur is through the application offorce at a rapid rate, applied in a fashion which overwhelms the abilityof the cell to sense and otherwise communicate the magnitude of theexcess force caused by accelerated masses as well as accelerationoverwhelming various structural elements of the cell.

The first breakage mode may be contained through the use of physicalsupports placed adjacent the cell and in the line in which the force isreceived. These supports prevent the cell from overextension by a forceof magnitude greater than that which the load cell is able to withstand.

In the second breakage mode, the load cell may be cushioned or otherwiseprotected from rapid overwhelming forces through the use of mountingmeans such as nonmetallic screws or bolts or metal bolts which have beencoated with a composition which will dampen the vibration created byrapid overwhelming force. Alternatively, a dampening shim may be placedbetween the load cell and the mounting base. The shim may be comprisedof any number of materials which will effectively dampen the vibrationscreated by the shock of the force incident to the cell. Dampeningmaterials which may be used in either instance, i.e. screws, bolts, orshims, include all compositions used to otherwise coat the wire leads.

WORKING EXAMPLES

When the load cell of the invention was compared to a MK16 system usinga strain gage based load cell and available from Weigh-Tronix thefollowing data resulted.

                  TABLE 1                                                         ______________________________________                                                     INVENTION  CONTROL                                               ______________________________________                                        Resolution     1,000,000/SEC                                                                              50,000/SEC                                        Accuracy       10,000 d OIML                                                                              3,000 d OIML                                      Creep (R-60)   25 PPM       70 PPM                                            Zero Return    30 MS        250 MS                                            Power          >10 MW       592 MW                                            Speed of Response                                                                            1X           3X                                                Shock test     1/2 Capacity 6"                                                                            1/2 Capacity 6"                                   (in Scale)                                                                    ______________________________________                                    

The linearity error test of the invention was also run on a 25 kg loadcell at temperatures of -10° , 5° , 20° , and 40° C. in accordance withOrganisation Internationale De Metrologie Legale and the protocolprovided therein, (see Page 10, Table 2, Class C R-60 (Ed. 1991)). Theresults of the analysis may be seen in FIG. 11.

Applications

The claimed invention may be used in any bending beam configuration. Thebeams may be attached rigidly to a base or levered by a fulcrum ormixed, fixed or supported fulcrum systems. The load cell may besubjected to compressive or tension forces or both through stress on theload cell. In systems with two or more beams, where the invention isaffixed between the beams, the beams may both be of fixed attachment tothe base. Alternatively, one beam may be of fixed attachment to the basewith the other beams fixedly attached to the first beam.

The invention may also be used in multiple beam systems, which do notprovide a rigid attachment of the beams to the base. In thisapplication, for example, multiple parallel beams may be positioned orsuspended beneath a base, with the claimed load cell placed in linebetween the beam and the base. Such an application might be seen insingle point scales such as scales used to weigh produce, livestock andthe like.

The invention may also be used to measure objects of great mass such astrucks, or large structures such as houses or buildings. In this case,the beam would be fixed upon two fulcrums positioned at either end ofthe beam with the claimed cell attached at a joint along the beam. Whenstressed, the claimed invention senses the forces required to deflectthe sensing beam.

Further, the claimed invention may be used in multiple fulcrum systemssuch as deflecting plates which are attached rigidly at one or moreedges, or levered by fulcrums.

The claimed invention may be used between two axially loaded springssuch as in coiled suspension spring applications. These types of systemsmay be seen in vehicle support applications or vibration isolationsystems, e.g. shock absorbance. In these applications, the force to beevaluated may be sensed independent of interference and in environmentswhere the force is constantly changing in magnitude.

In suspension systems, such as those which deflect when stressed, theclaimed invention may be used with multiple springs which through axialmovement provide compressive and tensile action on a load cell placedvertically in line with springs which are positioned at an angle withrespect to the line between the load and base. By positioning a loadcell between two sets of springs, chains or other flexible elements,load bearing mechanisms may be used to sense force in large scaleapplications such as suspended hoists.

The invention may also be used to sense measure or sense variations inpressure. Either absolute or differential pressure may be measured bypositioning the claimed load cell in line between two platforms, eachplatform being affixed to a containing spring system, such as bellows.In an absolute pressure system, for example, the load cell may besubjected to compressive forces created by an expanding bellows and asupportive or reactive bellows which are not pressure sensitive butmerely track or gauge displacement.

In differential pressure measurement, the load cell of the invention maybe positioned in line with opposing bellows or diaphragms, which are inturn positioned between opposing fluid ports. In summation, pressuremeasurement, the load cell of the invention may be placed in linebetween two opposing fluid sources.

The invention may be used to sense momentary variations in acceleration,such as laboratory applications such as non-repeatable experiments whereany effects created by hysteresis are to be eliminated. For example,destructive testing (e.g. automobile crash tests), explosives testingand the like are all applications which may use a system having a massattached outside of, or between, two load bearing elements with the loadcell of the invention also attached between the load bearing elements.

Torsion or moment sensing may also be accomplished by the load cell ofthe invention. In applications such as vehicle axles, motor output, andthe like (any movement which will create torque), the relative force maybe sensed by the load cell of the invention by attaching the load cellbetween the torque creating element and an axially aligned reactivetorsion element.

Any number of other applications such as torsional bending and the likemay be possible with the invention. The design of the load cell has beenfound to provide great variability in application and design through anynumber of mechanisms where the sensing of force absent material effectsor environmental interferences is desired.

The above discussion, examples and embodiments illustrate our currentunderstanding of the invention. However, since many variations of theinvention can be made without departing from the spirit and scope of theinvention, the invention resides wholly in the claims hereafterappended.

We claim as our invention:
 1. A force sensing load cell comprising:(a) aload cell body having an interior opening defined by an upper wall and alower wall joined by first and second side walls; (b) a base positionedwithin said opening and affixed to at least one of said opening walls;(c) a first cantilever beam affixed to said base; (d) a secondcantilever beam affixed to said base; (e) a first transducer comprisingtwo parallel tines joined together at their ends, said first transduceraffixed between said first cantilever beam and said base; (f) a secondtransducer comprising two parallel tines joined together at their ends,said second transducer affixed between said second cantilever beam andsaid base; and (g) means for sealing said load cell body interioropening.
 2. The load cell of claim 1 wherein said first and secondtransducers comprise quartz crystal double-ended tuning fork resonators.3. The load cell of claim 1 wherein said sealing means comprisessilicon.
 4. The load cell of claim 1 wherein said load cell interioropening comprises a front side and a back side and said sealing meanscomprises first and second silicon sealing windows, said first sealingwindow positioned over said load cell interior opening front side andsaid second sealing window is positioned over said interior opening backside.
 5. The load cell of claim 4 wherein said first and secondcantilever beams are positioned parallel to each other within the planeof the load cell interior opening and parallel to said base.
 6. The loadcell of claim 4 wherein said second sealing window comprises a plate. 7.The load cell of claim 1 wherein said first and second transducers areconnected by electrical leads, said leads inserted through said sealingmeans.
 8. The load cell of claim 7 wherein said electrical leadscomprise a dampening coating.
 9. A force sensing load cellcomprising:(a) a load cell body having an interior opening defined by anupper wall and a lower wall joined by first and second side walls saidinterior opening having a back end and a front end; (b) a basepositioned within said load cell interior opening and affixed to saidopening lower wall said base comprising a load beam adjoining saidopening upper wall; (c) a first cantilever beam, said first cantileverbeam affixed to said base; (d) a second cantilever beam, said secondcantilever beam affixed to said base; (e) a first piezoelectric quartzcrystal transducer comprising two parallel tines joined together attheir ends, said first transducer affixed between said first cantileverbeam and said load beam; (f) a second piezoelectric quartz crystaltransducer comprising two parallel tines joined together at their ends,said second transducer affixed between said second cantilever beam andsaid load beam; and (g) means for sealing said load cell body interioropening.
 10. The load cell of claim 9 wherein said load cell interioropening comprises a front side and a back side and said sealing meanscomprises first and second silicon sealing windows, said first sealingwindow position over said load cell interior opening front side and saidsecond sealing window is positioned over said interior opening backside.
 11. The load cell of claim 10 wherein said second sealing windowcomprises a stop plate.
 12. The load cell of claim 9 wherein said firstand second cantilever beams are positioned parallel to each other withinthe plane of the load cell interior opening and parallel to said loadbeam.
 13. The load cell of claim 9 wherein said first and secondtransducers are connected by electrical leads, said leads insertedthrough said sealing means.
 14. The load cell of claim 13 wherein saidelectrical leads comprise a dampening coating.
 15. The load cell ofclaim 9 wherein said first and second transducers are encapsulated andsaid sealing means comprises silicon gel inserted into said load cellbody interior opening.
 16. A force sensing load cell comprising:(a) amonolithic load cell body having an interior opening defined by an upperwall and a lower wall joined by first and second side walls saidinterior opening having a back end and a front end; (b) a basepositioned within said opening and affixed to said opening lower wallsaid base comprising a load beam adjoining said opening upper wall; (c)a first cantilever beam, said first cantilever beam affixed to saidbase; (d) a second cantilever beam, said second cantilever beam affixedto said base; (e) a first piezoelectric quartz crystal transducercomprising two parallel tines joined together at their ends, said firsttransducer affixed between said first cantilever beam and said loadbeam; (f) a second piezoelectric quartz crystal transducer comprisingtwo parallel tines joined together at their ends, said second transduceraffixed between said second cantilever beam and said load beam; (g) afirst sealing window affixed to said interior opening front end, saidfirst sealing window shaped to conform to the opening; and (h) a secondsealing window affixed to said interior opening back end, said secondsealing window shaped to conform to the walls defining the interioropening.
 17. The load cell of claim 16 wherein said first and secondcantilever beams are positioned parallel to each other within the planeof the load cell interior opening and parallel to said base.
 18. Theload cell of claim 16 wherein said first and second transducers areconnected by electrical leads, said leads inserted through said secondseal.
 19. The load cell of claim 18 wherein said electrical leadscomprise a dampening coating.
 20. The load cell of claim 16 wherein saidsecond sealing window comprises a plate.